Kevin is right - I think. We do have a header tank in the loft, but last winter for some reason the water in it was getting very (very!) hot. There is no lid on the tank, so it steams. Steam into a cold (in winter) loft results in condensation, which if left will probably cause damp. So something needs doing, just not sure what ATM.
It's a 1960's house, originally with a back boiler (behind the fireplace) and electric immersion heater. Gas central heating was added in the late 60's and the original boiler soldered on till about 2001. The header tank in the loft definately didn't get hot with that. That old boiler finally packed up and was replaced by the Baxi which we still have. Initially I don't remember the tank 'steaming' and I'm pretty sure I would have noticed - there is a lot of junk in the loft including the Christmas decorations which come down and go up once a year.
We had a small extension built about 3 years ago, and the Baxi was moved to accommodate the new kitchen layout. Then last winter I noticed the 'steaming' in the loft. Now the Baxi has packed up, but at least we still have the (50 year old!) immersion heater for hot water, and no urgent need to get the central heating working - at least for a month or two.
I like the sound of the System heater - it's not a combi so does heat a tank full of hot (tap) water, and doesn't require a header tank so that solves the 'steaming' loft issue at the same time. We have had multiple CH pumps pack u over the years due to the very hard water here, so having the pump integrated into the boiler is a bit of a concern.
I only posted my woes to try and put some context into the prices being touted to moggy though.
A few things ring alarm bells there. Firstly, hard water shouldn't have much impact on the life of the primary side of the system. You fill it once, and once the calcium in that batch of water has been deposited somewhere, you don't get any more forming. If you are getting issue due to calcium building up, then the system must be topping itself up and constantly therefore adding more calcium.
If the header tank is getting hot then the system could be boiling due to a faulty boiler thermostat, poor circulation around the boiler or perhaps the system has been plumbed in such a way that water can get pumped over the air vent pipe through the header tank. That will cause evaporation, and topping-up, and perhaps your calcium issues?
I'm guessing the boiler move resulted in such an issue that wasn't spotted at the time?
Going to a sealed system might well solve the issues, if not simply because you notice when topping up is required because it has to be done manually! It might be a lottery on a system that old as to whether it is happy with the increased pressure, mind.